The present invention relates to a wiring substrate and a manufacturing method thereof, and more particularly, to a wiring substrate having a low inter-layer connection resistance, provided with reliability in terms of electric insulation and reliability in terms of electric and mechanical connection, and suitable for a multi layer structure and a method for manufacturing such a wiring substrate.
It is important for a wiring substrate which is so formed in a multi layer structure as to highly densely mount semiconductor chips of LSI and the like that a plurality of wiring layers formed at fine wiring pitches can electrically be connected to one another at low connection resistance.
For that, in place of a conventional multi layer wiring substrate employing an inter-layer connection manner by through holes, a first technique and a second technique for enabling any electrode of a multi layer wiring substrate to be inter-layer-connected at any wiring layer position have already been disclosed in Japanese Unexamined Patent Publication No. Hei 6-268345 and Japanese Unexamined Patent Publication No. Hei 10-335526, respectively.
Hereinafter, the summary of both techniques will be described.
The First Technique
As it will be described with reference to FIG. 11, the first technique is to connect only necessary inter-layers by filling a conductive paste in inner via holes of a multi layer wiring substrate.
The outline of the connection process will be described with reference to FIG. 11(a) to FIG. 11(d).
As illustrated in FIG. 11(a), a through hole 202 is formed in a porous substrate 201 having void 302 and then as illustrated in FIG. 11(b), the through hole 202 is filled with a conductive paste 203 containing a conductive filler and a thermoplastic resin.
As illustrated in FIG. 11(c), copper foils 204 are overlaid on both side of the porous insulation substrate 201 and the insulation substrate 201 and the copper foils 204 are thermally press-bonded to cure the conductive paste 203 and both copper foils 204 are electrically connected through the conductive paste 203 and as illustrated in FIG. 11(d), the copper foil 204 are patterned in desired patterns to form wiring layers 205.
In the thermal press-bonding process in FIG. 11(c), the conductive paste 203 is pressurized in the thickness direction and the resin of the conductive paste 203 penetrates the voids 302 of the porous substrate 201. Consequently, the packed density of the conductive particles is increased and the conductive particles are brought into contact with one another to actualize the conductivity and connect wiring layers.
However, in case of the first technique, since the insulation substrate 201 is porous, as illustrated in FIG. 12(a), voids 302 exist in the peripheral parts of the position where the through hole 202 is formed. Attributed to the voids 302, recesses 304 recessed outward in the inner walls of the through hole 202 are formed.
When the through hole 202 having such recesses 304 is filled with the conductive paste 203 and the copper foils 204 are overlaid and thermally press-bonded, as illustrated in FIG. 12(b), the conductive paste 203 flows in the recesses 304 and causes so-called paste bleed 305, which is penetration of the insulation substrate layer 201 with the conductive paste.
In case of the first technique, since an electric field tends to be converged upon the paste bleed 305 and electric short circuiting to a wiring layer beside is easily caused attributed to the outward projected shape of the past bleed, the first technique leaves room for improvement in terms of insulation reliability.
The Second Technique
In case of the second technique, a thermosetting resin-containing insulation layer is disposed between an upper and a lower wiring patterns. The insulation substrate has a via containing a conductive paste (containing a metal powder, a binder, a solvent and the like). The binder and the solvent are removed by heating the insulation substrate. Consequently, gaps are formed among metal powder particles in the via. High pressure is applied to the thermosetting resin before complete curing to penetrate the via with the organic resin of the insulation substrate and to fill the gaps among the metal powder particles with the organic resin.
However, in case of the second technique, although there occurs no so-called paste bleed in which the conductive paste penetrates the insulation substrate, the loading ratio of the metal powder in the voids in the via is about 65% or higher and the voids are about 35% or lower. The voids are entirely filled with the organic resin and the resin is cured.
For that, no void is left in the entire via and the elasticity modulus as a whole is increased and the hardness is high. Therefore, even if mechanical stress is applied to the wiring substrate by heat in the thickness, direction, the entire body of the wiring substrate is hardly expanded or contracted responding to the stress and the wiring layers tend to be easily disconnected with the metal powder in the via. As a result, there is left room for improvement in terms of the reliability of electric and mechanical connection between wiring layers.
Consequently, the present invention mainly object to provide a wiring substrate with improved reliability of wiring layers in terms of insulation and connection and to provide a method for manufacturing such a wiring substrate.
(1) A first wiring substrate of the present invention comprises two or more wiring layers, insulation layers disposed between the foregoing neighboring wiring layers and containing an organic resin, and vias formed in the insulation layers for connecting the wiring layers, wherein the vias contain a plurality of functional substances and surrounded with voids each of which comprises first voids containing the organic resin from the insulation layers and second voids containing a gas.
For that, in the first case of the present invention, so-called paste bleed, which is penetration of an insulation layer with a conductive paste, does not occur and further, the elasticity modulus is low and the wiring substrate is made soft since the voids in a through holes are not all filled with an organic resin and some voids containing a gas such as air but not being filled with the organic resin exist, the elasticity modulus is low and the flexibility is improved in the entire via. Therefore, even if mechanical stress is applied to the wiring substrate by heat in the thickness direction, the via easily follow expansion and the contraction of the entire body of the wiring substrate responding to the stress and the upper and the lower wiring layers tend to be hardly disconnected with the functional substances such as a metal powder in the via and as a result, the reliability of electric and mechanical connection between the upper and lower wiring layers is heightened.
(2) A second wiring substrate of the present invention comprises two or more wiring layers, insulation layers disposed between the foregoing neighboring wiring layers, and vias formed in the insulation layers for connecting the wiring layers, wherein each of the insulation layers is a film bearing an adhesive layer containing an adhesive in at least one face and the vias contain a functional substance and the adhesive same as that in the adhesive layer and penetrating the voids surrounded the functional substance and existing in the voids.
In the second case of the present invention, paste bleed does not at all takes place and since the voids are formed in the surrounding of the functional substance contained in the vias and the adhesive of the adhesive layers penetrates the voids, no clear interface exists between the vias and the adhesive layers. Therefore, separation in an interface of a via and an adhesive layer hardly occurs and the reliability in terms of connection is heightened. A wiring substrate consequently provided has high reliability in insulation between neighboring wiring layers and high reliability in electric and mechanical connection between both upper and lower wiring layers.
(3) In the respectively improved wiring substrates of the first and the second of the present invention, the second voids are preferably formed selectively in agglomeration parts in the functional substance.
Owing to that, the second voids surrounded with the agglomeration parts of the functional substance are hardly degassed and the organic resin or the adhesive are made difficult to penetrates the voids, the low elasticity modulus is stably kept for a long duration and thus the reliability in the electric and mechanical connection of the upper and the lower wiring layers is heightened.
(4) In the respectively improved wiring substrates of the first and the second of the present invention, the second voids are preferably formed as to have the average volume smaller than the average volume of a plurality of respective functional substances forming at least the agglomeration parts.
Owing to that, even is a high mechanical stress is applied to the vias by expansion or contraction of the wiring substrates, the agglomeration state of the functional substances is kept well and thus the retain-ability of the second voids by the functional substances is heightened to make the second voids hardly crushed and to keep the low elasticity modulus for a long duration by the second void. In other words, the wiring substrates provided have the reliability in the electric and mechanical connection of the upper and the lower wiring layers kept stably for a long duration and for that, to keep the average volume in such a manner is preferable.
Incidentally, the foregoing wiring layers include un-patterned wiring layers and patterned wiring layers (wiring patterns and circuiting patterns).
Incidentally, the concept of xe2x80x9clayerxe2x80x9d of the foregoing insulation layers and wiring layers, is not restricted in the thickness.
Further, the wiring substrate include not only a substrate on which semiconductor chips such as LSI and the like and other electronic parts are mounted but also a substrate in the stage before mounting and a substrate on which no electronic part is to be mounted but wirings are to be formed.